Cant hurt and when you upgrade which you will, you will already be one step ahead. Smaller tank is harder to keep since any little change can effect the tank. With an Apex you can monitor those changes and be on top of it.
Personally, I wouldn't bother with an Apex for that size of tank... or so I tell myself, but I just put a GHL on my IM 20.
As long as you're only using the temp and PH probes, there's plenty of room in the AIO chamber. Even if you have the ORP and conductivity probes, you're still not talking about much real estate. It's only once you add the flow meters and Tridents and such that you need to start thinking about the footprint of everything.
I have an Apex on my Nuvo 20 and one on my Nuvo 14. I like the automation for lighting, ato, pH and heating. They are both in my office so I can check on them anytime. All the equipment easily fits in the cabinet along with a 5 gallon ato reservoir.
In my opinion you can never go wrong with a controller. I will also say they are not needed. I think once you have one you will never want to run a tank without one. You can always search local groups for an apex classic, but you wouldn’t regret just buying one either.